1. Field of the Invention
This invention generally relates to electrochemical batteries and, more particularly, to a method for the synthesis of iron hexacyanoferrate (FeHCF, Na1+xFe2(CN)6 (x=0 to 1)).
2. Description of the Related Art
Modern rechargeable lithium batteries have triggered the portable electronic device revolution due to their high power density, long cycle life, and overall reliability. The rechargeable lithium battery consists of a cathode (positive electrode) and anode (negative electrode), separated by a lithium ion (Li+)-permeable membrane. A solution or polymer rich in Li+ is employed to permit facile migration of lithium ions between the positive and negative electrodes. Common positive electrode materials include transition metal oxides such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and lithium iron phosphate (LiFePO4), in addition to various derivatives. Within these materials, Li+ can travel within the interstitial space(s) both freely and reversibly. In general, metallic lithium, alloys, and carbonaceous materials can be utilized as the negative electrode. When the rechargeable lithium battery does not include any metallic electrode, it is referred to as lithium-ion battery (LIB). During the discharge process in a lithium-ion battery, lithium ions are extracted from the negative electrode and subsequently inserted into the positive electrode. At the same time, electrons pass through an external circuit from the negative electrode to the positive electrode to generate electric power. During the charge process, ions and electrons move along the reverse directions and are ultimately restored to their original locations.
Although LIBs have been employed successfully over a broad range of commercial applications, the issue of lithium supply, as it applies to both strain on natural reserves and potential fluctuations in price, has motivated the development of a low-cost, rechargeable battery technology as an alternative to LIB. In light of this, sodium-ion batteries (NIBs) have increased in popularity due primarily to the fact that sodium has similar properties to lithium, but also boasts the benefits of both reduced cost and virtually unlimited availability. However, analogous to LIBs, NIBs require appropriate Na+ host materials. Indeed, significant effort has been devoted towards the direct duplication of Li+ host structures for application as Na+-host electrode materials for NIBs. For example, NaCoO2, NaMnO2, NaCrO2, and Na0.85Li0.17Ni0.21Mn0.64O2 with similar layered-structure to LiCoO2, have been developed for NIBs. Similarly, Co3O4 with a Spinel structure, Na3V2(PO4)3 with a NASICON structure, and NaFePO4 with an Olivine structure have been employed in sodium batteries. In addition, sodium fluorophosphates, such as Na2PO4F, NaVPO4F, and Na1.5VOPO4F0.5 have also been utilized as the positive electrode for sodium batteries.
Overall, it appears impractical to simply adopt conventional Li+ host materials and structures as Na+ host compounds, since sodium ions are larger than lithium ions and, consequently, severely distort the structures of Li+ host compounds during the intercalation process. In response to this, it is critical to develop new Na+ host materials with large interstitial space(s) through which sodium ions can migrate, both easily and reversibly.
It is widely known that Na+ and others ions readily intercalate into metal hexacyanometallate compounds. In general, Prussian Blue and its analogues belong to a class of mixed valence compounds called transition metal hexacyanometallates (TMHCMs). In addition to energy storage, these materials have been identified for applications that include electrochromics, chemical sensors (for nonelectroactive cations and hydrogen peroxide, for example), and advanced biosensors. In general, TMHCMs are characterized by a general formula corresponding to AxM1mM2n(CN)6 where M1m+ and M2n+ are transition metals with different formal oxidation numbers (m and n). In many cases, the TMHCMs can sequester a variety of different ions (“A”=Na+, K+, NH4+, Co2+, Cu2+, for example) as well as various amounts of water (H2O) within the crystal structure.
FIG. 1 is a schematic diagram depicting the crystal structure of TMHCM, which exhibits an open framework and is analogous to that of the ABX3 perovskite (prior art). M1m+ and M2n+ metal ions are in ordered arrangement on the B sites. The large, tetrahedrally coordinated “A” sites can host both alkali and alkaline earth ions (Ax) in addition to species such as ammonium (NH4+) and H2O. The number of alkali (or alkaline earth ions) in the large cages of this crystallographically porous framework may vary from x=0 to x=2, which is dependent on the respective valence(s) of M1 and M2.
Previously, TMHCFs with large interstitial spaces have been investigated as cathode materials for rechargeable lithium-ion batteries,1,2 sodium-ion batteries,3,4 and potassium-ion batteries.5 In aqueous electrolytes containing appropriate alkali metal ions or NH4+, copper and nickel hexacyanoferrates [(Cu,Ni)-HCFs] demonstrated robust cycling with 83% capacity retention after 40,000 cycles at a charge/discharge current density of 17C (1C=150 milliamps per gram).6-8 In spite of this, the materials demonstrated low corresponding capacities and energy densities due to the fact that (1) only one sodium-ion can be inserted/extracted into/from per Cu-HCF or Ni-HCF formula, and (2) the TMHCF electrodes required operation below 1.23 V due to the electrochemical stability window for water. The electrochemical stability window of a substance is the voltage range between which the substance is neither oxidized nor reduced. This range is significant in terms of electrode efficiency since water becomes electrolyzed outside this range, which consequently cannibalizes the electrical energy intended for another electrochemical reaction.
In order to compensate for these shortcomings, manganese hexacyanoferrate (Mn-HCF) and iron hexacyanoferrate (Fe-HCF) were used as cathode materials in non-aqueous electrolyte.9,10 For batteries containing a metallic sodium anode, Mn-HCF and Fe-HCF cathodes demonstrated capacities ˜110 milliamperes hours per gram (mAh/g) when cycled between 2.0 V and 4.2 V.
Despite the significant potential for TMHCFs in secondary battery applications, it has proven extremely difficult to directly synthesize Na1+xFe2(CN)6 (x=0 to 1) through a conventional precipitation process. Specifically, upon addition of Fe2+-containing solution to a solution of ferrocyanide [Fe(CN)64−], Fe2+-ions are almost instantaneously oxidized to Fe3+ ions, yielding a corresponding dark blue precipitate consistent with Na1−xFe2(CN)6 (x=0 to 1).
FIG. 2 is a graph depicting the capacity of a Na1+xFe2(CN)6 (x=0 to 1) electrode, the synthesis of which was unsuccessful using direct mixing of a solution of a Fe2+ source containing Fe(CN)64−. During the first charge, the material generated a modest capacity of 40.85 milliamperes hours per gram (mAh/g), which corresponds to 0.52 Na+ ions per formula unit (fu). Furthermore, thermal gravimetric analysis (TGA) confirmed 2.79 water molecules existing per fu. Thus, the molecular formula can be calculated as Na0.52FeIII[FeII(CN)6]1−y.2.79H2O (y<1), rather than the intended Na1+xFe2(CN)6 (x=0 to 1). Finally, the small Na+ content confirms that a certain proportion of Fe(II) species in Fe(CN)64− was also oxidized during the process. Based upon these results, it would appear as if Na1+xFe2(CN)6 (x=0 to 1) cannot be readily synthesized using a conventional precipitation method.
Hu et al. reported a hydrothermal method to synthesize K2Fe2(CN)6 from K4Fe(CN)6 in a neutral solution.11 However, some practitioners are skeptical of this reported method of synthesizing Na2Fe2(CN)6 due to the fact that sodium ions are smaller than potassium ions and, therefore, harder to retain in the large interstitial space of iron hexacyanoferrates (Fe-HCFs), Further, the reaction is sensitive to solution pH.12-14 In response, Lu et al. demonstrated a modified hydrothermal reaction process to successfully synthesize sodium iron hexacyanoferrate (Na2Fe2(CN)6) directly (U.S. Ser. No. 14/067,038, which is incorporated herein by reference). According to this method, reducing agents are included in the reaction solutions to protect Fe(II) from oxidation during the hydrothermal process.    [1]V. D. Neff, “Some Performance Characteristics of a Prussian Blue Battery”, Journal of The Electrochemical Society 1985, 132, 1382-1384.    [2]N. Imanishi, T. Morikawa, J. Kondo, Y. Takeda, O. Yamamoto, N. Kinugasa, and T. Yamagishi, “Lithium Intercalation Behavior Into Iron Cyanide Complex as Positive Electrode of Lithium Secondary Battery”, Journal of Power Sources 1999, 79, 215-219.    [3] Y. Lu, L. Wang, J. Cheng, and J. B. Goodenough, “Prussian Blue: A New Framework for Sodium Batteries”, Chemical Communications 2012, 48, 6544-6546.    [4]L. Wang, Y. Lu, J. Liu, M. Xu, J. Cheng, D. Zhang, and J. B. Goodenough, “A Superior Low-Cost Cathode for a Na-ion Battery”, Angewandte Chemie International Edition 2013, 52, 1964-1967.    [5] A. Eftekhari, “Potassium Secondary Cell Based on Prussian Blue Cathode”, Journal of Power Sources 2004, 126, 221-228.    [6]C. D. Wessells, R. A. Huggins, and Y. Cui, “Copper Hexacyanoferrate Battery Electrodes with Long Cycle Life and High Power, Nature Communications 2011, 2, Article number: 550.    [7]C. D. Wessells, S. V. Peddada, R. A. Huggins, and Y. Cui, “Nickel Hexacyanoferrate Nanoparticle Electrodes for Aqueous Sodium and Potassium Ion Batteries”, Nano Letters 2011, 11, 5421-5425.    [8]C. D. Wessells, S. V. Peddada, M. T. McDowell, R. A. Huggins, and Y. Cui, “The Effect of Insertion Species on Nanostructured Open Framework Hexacyanoferrate Battery Electrodes”, Journal of The Electrochemical Society 2012, 159, A98-A103.    [9]T. Matsuda M. Takachi, and Y. Moritomo, “A Sodium Manganese Ferrocyanide Thin Film for Na-ion Batteries”, Chemical Communications 2013, 49, 2750-2752.    [10]S-H. Yu, M. Shokouhimehr, T. Hyeon, and Y-E. Sung, “Iron Hexacyanoferrate Nanoparticles as Cathode Materials for Lithium and Sodium Rechargeable Batteries”, ECS Electrochemistry Letters 2013, 2, A39-A41.    [11]M. Hu and J. S. Jiang, “Facile Synthesis of Air-Stable Prussian White Microcubes via a Hydrothermal Method”, Materials Research Bulletin 2011, 46, 702-707.    [12] S-H. Lee and Y-D. Huh, “Preferential Evolution of Prussian Blue's Morphology from Cube to Hexapod”, Bulletin of the Korean Chemical Society 2012, 33, 1078-1080.    [13] M. Hu, J-S. Jiang, C-C. Lin, and Y. Zeng, “Prussian Blue Mesocrystals: An Example of Self-Construction”, Cryst Eng Comm 2010, 12, 2679-2683.    [14]M. Hu, R-P. Ji, J-S. Jiang, “Hydrothermal Synthesis of Magnetite Crystals: From Sheet to Pseudo-Octahedron”, Materials Research Bulletin 2010, 45, 1811-1815.    [15] R. K. Patil, S. A. Chimatadar, and S. T. Nandibewoor, “Oxidation of Thiosulfate by Hexacyanoferrate(III) in Aqueous Perchloric Acid Medium—A Kinetic and Mechanistic Study”, Indian Journal of Chemistry 2009, 48A, 357-361.
It would be advantageous if Na1+xFe2(CN)6 (x=0 to 1) containing a high concentration of sodium ions could be synthesized using a simple, low-cost process.